When buying grinders, buffers, grinding wheels, and buffing supplies, the more one understands about using this equipment, the better the buying choices will be.
Use this formula for calculating surface speed of wheel in SFPM. (Surface Feet Per Minute) or RPM
The scope of information for grinding wheels is too large to try to cover here, instead we will focus on the type of wheel and working procedure most commonly used in off-hand grinding.
For off-hand grinding steel with vitrified wheels, a surface speed (SFPM) of 4500 to 6000 is recommended. In off-hand grinding, the pressure employed in forcing the work against the wheel, or the method of applying the work, influences the grade selection. The more severe the pressure and application conditions, the harder should be the wheel in order to give economical wheel life. This is the reason the typical wheel used on a pedestal grinder is of a fairly hard grade.
For example, if you are using an 8" grinding wheel running at 1800 RPM, using the calculator above, the surface speed would be approx 3700 SFPM, which is slightly below the recommended speed. If the grinder was running at 3600 RPM, the same calculation would give approx. 7500 RPM, which is too fast.
It is better to be slower than recommended rather than faster because slower speeds make the wheel act slightly softer, which makes the wheel actually cut faster and cooler. This is especially true if you are using the M grade wheel that usually ships with the grinder. (Slower wheel speeds require higher motor torque, which is why you never see cheap grinders running at slow speeds).
If you are only grinding hard steel, such as high speed steel, the softer wheels perform well, but require careful usage and proper dressing, as plunging a piece of soft steel into the softer wheel will degrade the even surface, and require additional truing of the wheel.
How do I select the correct grinder for my grinding applications ?
First, decide what type of grinding you will be mostly doing. If you are going to be sharpening small hardened tools, such as lathe bits, drills, etc., you will probably only need the smaller horsepower grinder. Too slow a speed with a soft wheel will consume abrasive without corresponding useful work. Excessive speed with a hard wheel may result in a hard grinding action, wheel glazing, and overheating the work.
If tool heating is a problem, then the 1800 rpm is a better choice. Someone told me once that quenching the tool frequently would make the 1800 rpm unnecessary. This is just not true. The problem is heat WHILE you are grinding. On some tools, if you let the metal reach a certain temperature, the damage has already been done, no matter how much you quench.
If you are sharpening high speed steel, which has a high "red hardness" heating even up to turning red then quenching does no appreciable harm to the cutting edge. (Usually only your fingers suffer).
The Baldor model 7312D, for example, is a two speed model. It runs both 1800 rpm, and 3600 rpm. If you need both a high speed, and a low speed grinder, this is your best choice. It is a 7 inch, 1/2 hp unit, with cast iron tool rests, and exhaust guards.
If you are going to be doing heavier grinding, then select a larger horsepower grinder.
It is important that you not stall or slow down the grinder with your grinding application, as the
wheel cuts best at a consistent rpm. If you have mounted a softer wheel so as to grind hardened tools, it will "break down" quicker if the grinder is too small and slows down under load.
If your working environment is not adequately lighted, then choose a model that comes fitted with lighted eye-shields.
The addition of the water pot and tool tray makes using the grinder safer and easier. Hint: Place a plastic liner in the cast iron water pot to prevent rust and make changing or adding water easier.
If you do not have a proper bench to mount the grinder onto, then add the grinder stand.
This is especially true for the larger grinders.
If you are going to use the grinder mostly for general Deburring of items, then consider selecting a model with one grinding wheel and one wire brush. This is very handy to remove the small burr left by the grinder. Also the wire brush can be used to remove scale and rust.
How do I select the correct Buffer for my applications ?
First, decide whether you need 1800 or 3600 RPM. Softer metals such as aluminum and brass need slower speeds, while harder metals such as chrome and stainless can be run at higher speeds. If you are buffing parts that have fairly sharp corners that you do not want to "round off", use a harder buff, or a faster speed, as faster speed makes the wheel act "harder". It is usually better to use a harder buff.
The higher horsepower buffers allow longer spindles, which give more buffing clearance for your part.
Talk to buffing experts, and people who do buffing themselves. Not everyone will share their "trade secrets", but you can glean a lot of information if you ask around.
The faster speed makes the buffing wheel act "hard". The plies do not separate as easily as at a slower speed, because the high surface speed creates more radial inertia. Use this high speed if you want the wheel to act more like a grinding wheel, or if you do not want the wheel to flare out.
The slower speed allows the plies to separate more easily, which will allow the wheel to conform to the shape of your part. If you are only doing light buffing, then one of the smaller models will do fine.
Proper buffing requires the motor not "pull down" or try to stall under heavy load, so if you are buffing
objects that may require heavy pressure, then a larger motor horsepower will be needed.
Why does my wheel "glaze" over ?
Sources of the trouble of the wheel loading or glazing are:
Incorrect wheel: Use coarser grain size, more open bond, or softer grade.
(Soft material=harder wheel, Hard material=softer wheel.)
Improper dressing: Keep wheel sharp with sharp dresser, use faster traverse dressing, and deeper dressing cut Dressing and Truing are two different processes. Dressing is to "sharpen" the wheel surface by exposing new cutting edges, while Truing is to make the wheel diameter concentric with the spindle rotation. Also, different wheels sometimes require special dressing tools and techniques. For example, the Norton SG (seeded gel) wheels require special diamond dressers, or you will actually make the wheel surface "dull" by dressing.
Faulty operation: Control feeds and speeds to soften action of wheel. A slower speed makes a wheel act softer.
Cheap wheel. All wheels are not created equal.
Lube: In special cases, you can successfully use "belt dressing" to lube the wheel.
How do I clean the compound from the buffing wheel ?
Unless you are going to use the wheel for a different compound, you only need to "loosen the plies" if the surface of the wheel is getting too "hard". This can be accomplished by forcing the end of a rod, such as 3/8 or 1/2 " dia cold roll steel into the rotating wheel until the wheel surface is "loosened" up.
It is better to have wheels designated for certain compounds, and to change the wheel when changing to different finishes.
It is better to not over do the "raking" of wheels. It only shortens the wheel life and makes a mess.
If you feel you need to "rake" the wheel, do the following:
Use a short length of coarse tooth stiff saw blade with duct tape handles on each end to protect your hands, or a commercial rake. Use this tool to "rake" the wheel until it is clean.
Also, "rakes" are available from most tool supply stores.
How do I dress a pedestal grinder wheel ?
There are several different dressing methods. The method you use depends on the application,
and to some extent, on the operator's skill.
Dressing stick. This is usually made of Silicon Carbide, Sintered Boron Carbide, or similar material.
Gesswein has several dressing stones: "TRU-STONE", DRESSING STONE-M, DRESSING STONE-C TYPE VS, and PBC. Each one of these has a particular application. Most machine shop supply or abrasive supply stores sells the stick type dressing stick. They are easy to use, and not very expensive. Another example is the Norton "NORBIDE" dressing stick.
These type of dressers do not get the wheel as sharp as the "star" or diamond dresser.
Star Dresser: These dressers are available from machine shop supply stores.
They are very easy to use, and will will quickly restore a sharp surface to a glazed wheel. The stars wear down, but are cheap and easy to replace. Care must be taken to dress carefully so as to not cause wheel run out. This type of dresser is not very good to "true" a wheel. It is mainly for sharpening "dressing" the wheel. This type of dresser is fairly easy to master.
Diamond Dressers: PREFERRED METHOD; Diamond dressers used for pedestal grinders need a good sized handle or they need to be mounted in a controlled movement device, otherwise they are hard to control. There are several types on the market. The type commonly referred to as a "T-Handle Diamond Strip Dresser", is the easiest to use. This type has a row of diamond points, allowing some control of material removed from the wheel. This type of diamond dresser is the preferred choice for the professional machinist, but a guide to control lateral movement is needed. A set-screw collar on the shank that can ride against the tool rest works will usually do the job. The single point type is best used with some sort of guide, as it is hard to get a flat smooth surface by hand. This can be accomplished by attaching a collar on the shank, which can be held against a straight surface as the tool is traversed slowly across the wheel. Another method is to make a block to hold the shank of the diamond tool, with a method of guiding against the tool rest so as to allow a smooth sideways movement of the diamond across the wheel face. This method takes some practice.
Why does my grinder vibrate even after I have dressed and trued the wheels?
The most frequent cause of vibration is wheel "run out". This run out is usually caused by improper wheel dressing. Make sure to dress evenly, and use some sort of a guide to allow you to traverse in a controlled straight line with the dressing tool.
If truing the wheel by dressing does not make the vibration go away, there are at least six possible culprits:
- The wheel itself is out of balance. This occasionally happens in manufacturing. You can test for this by standing the trued wheel on it's outside diameter on a level surface. The wheel should not roll from any point of rest on it's perimeter. If you think the wheel itself is out of balance, discard it.
- The spindle is either bent. or not machined concentric with the motor armature. You can check this both visually, and with a dial indicator. This is a common problem with low cost grinders, and imports. Also, check the spindle diameter against the hole in the wheel. There should be only enough clearance to allow the wheel to be slid onto the shaft with no play.
- The motor itself is out of balance. I have seen this frequently on lower cost grinders and imports. Sometimes even the bearings can be bad or worn, which will allow any motor or wheel imbalance to be amplified
- The fit between the wheel mounting hole and the spindle is loose or not concentric. It is common to use plastic bushings to adapt the wheel mounting hole down to the spindle diameter. If this is done by stacking several bushings inside each other, the combined clearances can cause a problem. Also, I have seen these bushings themselves manufactured not concentric. The most accurate method is to turn your own bushing from Delrin, Nylon, or similar plastic on the lathe. It only takes a few minutes. Turn the outside diameter and bore the inside diameter in one setup before parting the plastic bushing off. This will ensure concentricity. The bushing should be a light hand press into the wheel, and a snug slip fit onto the spindle of the grinder. It is commonly thought that it does not matter if the wheel is loose on the spindle, as dressing will true it anyway. THIS IS NOT TRUE. If the wheel is loose on the spindle, the whole mass of the wheel is moved off rotating center, and no amount of dressing will cure this.
Note: If the wheel wobbles side to side, it usually is not the problem, as this is a self balancing action, unless you have dressed the side of the wheel. (If you dress the side of the wheel, make sure you dress both sides evenly and completely.) It takes a special setup to successfully dress the sides of a grinding wheel on a pedestal grinder.
If you have a lot of wheel wobble, which makes using the side of the wheel difficult, check the quality of fit of the wheel flanges. The lower cost and import grinders usually do not have an adequate shoulder for the wheel flange to fit against, and the wheel flange itself may not be parallel. I have even seen this on more expensive grinders. The flange can be made parallel either by machining, or on a surface grinder. The quickest method is to lay the flange large diameter down, on the magnetic chuck of the surface grinder, and grind until the face that seats against the spindle shoulder cleans up. This usually only removes a small amount of material, but it really makes a difference. A small amount of run out of the flange will be amplified at the wheel diameter. Flange repair illustration:
- A combination of all the above. Each one by itself may not be the culprit, but combined inaccuracies can cause vibration.
- As a last resort, sometimes you can rotate the wheels in relationship to each other, to offset and cancel out any wheel weight imbalances induced in manufacturing.
** Make sure to UNPLUG the grinder before doing any work or changing wheels. **
Why is my buffed surface hazy?
When putting the last buff on your part, rotate it in the same direction the wheel is rotating. (Down towards the floor) This will produce a much better finish than if you rotate the part upwards, against the buffing wheel rotation. The reason for this, is that there are small fibers being “slung” outwards by centrifugal force, and if you rotate the part in the opposite direction of wheel rotation, they actually contact the part, creating a “haze”. When you rotate the part in the same direction as the buffing wheel is rotating, the last thing to “see” your part is the wheel surface doing the polishing.
How can I cool my wheel?
Buffs operating under constant heavy pressure must be ventilated in order to dissipate the heat generated.There are basically three types of buffing wheels:
1. Full Disk Buffs, which can be either loose or sewed.
2. Sewed Piece Buffs, which always are sewed.
3. Some sort of patented Folded Buff.
One of the advantages of a Folded Buff, is that the pockets tend to trap air, which has a cooling effect on the buff surface.
Full Disk Buffs may be ventilated by the process known as packing. This consists of assembling one or two disks of smaller diameter than the buff disk between each pair of buff disks, or at intervals as may be necessary.
Sewed Piece Buffs can be ventilated by inserting a few disks of fabric or washers of felt or leather between each section.
Remember to move your part back and forth sideways to prevent streaks caused by the "gaps" in the wheel if you are using one of the above methods of ventilation.
Other related guides
Buffing Wheels - Felt Wheels vs Cloth Wheels